Modems. Chapter 17. Modems are little devices to use the telephone to talk to other computers. Modem is an abbreviation for Modulator/ Demodulator Modems can be internal or external. Basic Knowledge. Modem Signals.
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Modems are little devices to use the telephone to talk to other computers.
Modem is an abbreviation for Modulator/ Demodulator
Modems can be internal or external.
Modems take analog signals and convert them into digital signals and vice versa.
The data that goes into the modem is called serial communication and is a series of 0’s and 1’s. This data is converted into bytes via the UART(Universal Asynchronous Receiver/Transmitter) so CPU’s can understand information
There are 2 ways of “chopping up” the incoming signals
The main way to serial ports communicate
The data is “chopped up” into 7 or 8 bit packets. 8-bit is most common
Each Packet starts with a start bit that tells the receiving modem that it is the beginning of a piece of information and it always is zero and ends with a stop bit that tells the receiver the packet is over.
Asynchronous communication packets have an optional parity bit used for error detection. Parity bits are rare
Asynchronous packet settings are described in a particular order: data bits, parity type, then stop bits.
Example: If a system uses 8 data bits, no parity, and 1 stop bit, you’d say 8-N-1.
Mainly for uploading and downloading programs to and from your computer.
Protocols are sets of definitions used by communication programs so that both computers are speaking the same language. There are many protocols that basically work the same way.
When a modem is going to send data with synchronous communication it sends a series of standardized bytes called sync bytes, which tell the receiving port that it is about to get information.
After sync bytes the sending modem adds a start-of-text(STX) marker then sends the data.
After data transmission the packet ends with an end-of-text(ETX) marker and error checking characters(ECC)
The receiver responds with an acknowledge if data is good or a no acknowledge if there is an error.
FTP’s and Baud Rate
File Transfer Protocol
FTP is used to enable you to exchange files with mainframes, mini’s, or other PC’s. It should be included in the software package that comes with your modem
Refer to pages 1046-1047 for variations of FTP.
Baud rate is the time that a modem uses as its carrier frequency
Baud rate speed is measured in bits per second
Example: If baud rate was 2400 and modulated at 2 bits per cycle the bps would be 4800
Flow Control (Handshaking)
Process by which 2 serial devices verify a conversation.
During data transfer there are 2 conversations when flow control is needed, Local(between modem and com port) and end-to-end(between modems).
Flow control can come in hardware and software. Look at pg. 1049 to learn about each.
CD – Carrier signal detected
RD or RX – Receive Data
TD or TDX – Transmit Data
DTR – Data Terminal Ready
DSR – Data Set Ready
RTS – Request To Send
CTS – Clear To Send
Commands of modems are known as the command set.
There is no standard for modem commands, but there is a main set that most companies follow. It is the famous AT command set.
AT commands are entered through the keyboard.
A list of AT commands is on pg. 1052-1053
AT – checks to see that modem is plugged in
ATH – Hangs up
ATM1 – turns the speaker on
ATM0 – turns the speaker off
ATD – dials a number
ATZ – resets the modem
Talking to serial ports and telephone lines
A phone line is guaranteed to handle speeds only up to 2400 baud
To verify you are getting the best quality is to call the phone line.
If the company thinks the voltage drop is too much, they will condition the line. Conditioning is adding a little voltage to the line to compensate for the voltage drop.
Talking to serial ports
Data communication equipment is the device that sends data. Data Terminal Equipment is the device that receives data. The connection between the 2 is called the handshake.
Chart on page 1054
Heart of many serial ports.
It takes parallel information from data buses and makes it serial information to be sent out to a modem.
It’s important to know what type of UART is in your COM ports. You can find out the type by checking the diagnostics tab/ More Info button under the Modems applet in the control panel under windows 9x. In Windows 2000 use the Phone and Modem Options applet.
For UART’s check pg. 1056
Fax technology was invented in 1842 by Alexander Bain
Modems send data in single dots which are converted to text.
4 different classes or groups of fax standards
Groups 1 and 2 were analog standards for fax modems operating at 300 baud.
Group 3 was standardized for digital facsimile devices to communicate over phone lines. Fax transmission had a max speed of 14400 bps.
Within group 3, there are several different classes that explain devices using different speeds and methods of communication.
Any good fax modem software can run G3
Chart of Fax standards on 1058
Standardized series of responses between two modems so they can determine optimal speed, data compression, error detection, etc.
Chart pg. 1059-1060
TAPI is a set of dynamic link libraries (DLL) that enables Windows 9x to perform telephone functions.
A good TAPI installation will configure the modem type, I/O address and IRQ, local phone number, long distance dialing codes, and etc. to enable it to use the modem.
TAPI is also used for answering machines, handle faxing, call forwarding, caller ID, and paging
Installation and Troubleshooting
Modems rarely fail, but problems usually are COM port problems
Set up and verify the com ports
Make sure you install nonconflicting I/O addresses and interrupts.
Check modems in device manager to verify the modem is working properly.
Windows 9x and 2000 PnP technology makes installation very easy. To do it manually use the add new hardware wizard.
Verify the Modem’s BIOS
Modern modems all have on board BIOS
A better term for BIOS would be command set
The BIOS is stored on the flash ROM of a modem.
BIOS can be corrupted easy by simply removing and reinserting the modem
Set Up the Correct Command Set
All that Windows communication programs need to handle a modem are the latest IFN files for your modem and version of Windows.
If Windows doesn’t verify the modem try a different brand.
For dial configuration use the phone and modem option and the control panel
Windows 2000 will not detect some internal modems so you must do manual installation.
When Modems Break
Check COM port and IRQ port
Program called modem doctor is very good for this
Modem doctor verifies COM ports and tests your modem and tells what kind of shape its in.
If your software says there is no modem
Make sure modem is looking at correct port
Check that there is no conflict with other devices
Reinstall all software and reconfigure your modem.